Differential activity and activation of Bacillus thuringiensis insecticidal proteins in diamondback moth, Plutella xylostella

Whole-crystal preparations from strains HD-1 and HD-133, activated Cry1Ab a nd Cry1C toxins as well as Cry1Aa, Cry1Ac, Cry1D, and Cry2Aa protoxins were tested for toxicity to 2nd-instar larvae of the diamondback moth, Plutella xylostella. Mortality data recorded after 2 and 5 days provided different results that were related to differential rates of solubilization, activati on, and degradation of insecticidal crystal proteins. The two most active p roteins are Cry1Ab and Cry1C, which are both present in HD-133. The Cry1Ab protoxin is activated within 2 days, whereas activation of the Cry1C protox in occurs between 2 and 5 days. HD-133 is more active than HD-1 immediately after infection and remains toxic over 5 days owing to the sequential acti vation of its crystal components. Solubility properties of crystals and rat es of activation of protoxins influence the overall toxicity of HD-1 and HD -133 to the diamondback moth.